Wood is a common commodity used in home building (frames and trusses), exterior construction (fencing, gazebos, trellising), indoor use (furniture, floors), in ground use (farm fencing, vineyard trellising, utility poles) and for use in marine environments (piling). Apart from naturally durable woods, all these applications require chemical protection of the wood from fungal, bacterial and insect attack, especially from termite attack.
Standards for wood protection have been developed either on a country or regional basis. These standards are subdivided into hazard classes based on the biological hazard involved and the commodity involved. For example, the Australian and New Zealand standards (AS1604 and NZS3640) provide the following Hazard classes:                Hazard Class 1: Insect Attack; wood in internal situation protected from weather.        Hazard Class 2: Termite Attack; wood in internal situation protected from weather.        Hazard Class 3: Fungal, insect and termite attack; wood in external situation above ground but subject to rain wetting.        Hazard Class 4: Fungal, bacterial, insect and termite attack; wood in ground, subject to rain wetting.        Hazard Class 5: Fungal, bacterial, insect and termite attack; wood in ground, subject to rain wetting. High hazard for utility poles etc.        Hazard Class 6: Marine organisms, fungal, bacterial, insect and termite attack in marine environment.        
For each Hazard Class, the standards define retention and penetration of the wood commodity required by preservative treatment. For example, Hazard Class 3 and above will normally require at least full sapwood penetration of the preservative chemical, whereas envelope treatments are acceptable for insect and termite protection in Hazard Classes 1 and 2.
The preservative may be included in a composition containing a carrier. Carriers range from water to non-aqueous carriers such as solvents or oils depending on their boiling point range.
Wood treated with aqueous preservative compositions increase the water content of the wood and cause swelling. A typical water-borne treatment has an uptake of 300 to 600 L/m3. These treatments are often referred to as providing “wet after” wood. Wet after wood will dry in service down to provide an equilibrium moisture content. In Australia and New Zealand, typical equilibrium moisture content is 15-18%. The drying of the wet after wood will subject the wood to shrinkage and checking which can affect the appearance of the timber. Furthermore, swelling or shrinkage in a wall frame or truss can lead to both structural and cosmetic defects in a building.
Non-aqueous formulations, such as Light Organic Solvent Preservatives (LOSP), provide timber that can be supplied at a moisture content equivalent to the equilibrium moisture content and are often referred to as providing “dry after” wood. A typical LOSP treatment has an uptake ranging from 30 to 50 L/m3. Unlike aqueous formulations, non-aqueous formulations do not swell the wood. However, non-aqueous formulations often contain high levels of volatile organic chemicals (VOCs) that can result in release of “greenhouse gasses” and odour being associated with the treated timber. Some non-aqueous formulations although initially assisting in penetration of the preserving agent into the wood can subsequently cause the preserving agent to bleed to the surface of the wood where it is then lost. Non-aqueous treatments, although not swelling the treated wood, are significantly more expensive than aqueous treatments.
Another problem with non-aqueous treatment occurs when the wood to be treated is infected with fungi causing, for example, “blue stain” or sapstain. In such infected wood, there is discolouration of the wood. Although the fungal infection does not adversely affect the strength of the wood, the infection causes rapid and uncontrolled uptake of non-aqueous preservative compositions into the wood. This is a real economic problem, particularly with expensive non-aqueous formulations used for dipping methods.
“Dry after” wood can also be achieved by redrying wood that has been water-borne treated but this is expensive and can result in timber degradation due to splitting and dimensional movement.
Some aqueous/glycol formulations can be used to form envelopes. However, these compositions are fundamentally polar and result in swelling of the wood.
Furthermore, neither aqueous nor non-aqueous formulations available allow control of penetration of the preserving agent to obtain an envelope of specific depth.
There is a need for new wood preserving formulations that allow controlled penetration of the preserving agent, do not result in a swelling of the wood or bleeding/leaching of the preservative and are more economical than formulations currently available.